1. Field of the Invention
The present invention concerns a lead frame for use in a semiconductor integrated circuit device and, more in particular, it relates to a lead frame suitable to use in a high pin count LSI package with an outer lead pitch of less than 0.5 mm.
2. Description of the Prior Art
Along with the progress in an improvement for the functions of semiconductor integrated circuits such as in microcomputers or gate arrays, high pin count technique has now been under development in plastic packages in which semiconductors are resin-encapsulated.
FIG. 3 shows an appearance of an existent lead frame used for the assembly of a QFP (Quad Flat Package), which is an existent general surface mounting IC plastic package. Details for the specification of the QFP are defined in the Standards of Electronic Machine Industry Association of Japan (EIAJ) in IC-74-4.
In FIG. 3, a lead frame 1 has a rectangular die pad 2 disposed at a central area thereof for mounting a semiconductor chip, around which leads 3 connected electrically with the semiconductor chip by way of lead wires such as gold wires are disposed. The die pad 2 is suspended by suspending leads 4. The lead 3 is divided into an inner lead part 3a which is a portion encapsulated with a resin and an outer lead part 3b which is a portion exposed to the outside when the package main body is molded. A boundary between the package main body and the outside is referred to as a mold line 5 (shown by a dotted line).
In the existent lead frame, a tie bar (also referred to as a dam bar) 6 for connecting the outer lead parts 3b to each other is disposed at a position to the outer circumference by 2 to 3 mm of the mold line 5 for preventing the resin from leaking outwardly between each of the leads upon molding. The outer circumferential portion of the lead frame 1 comprises an outer frame 7 and an inner frame 8, and guide holes 9 used upon molding for positioning are disposed to the outer frame 7. The lead frame 1 is generally made of an electroconductive material such as 42 alloy or copper alloy, which is molded by pressing or etching and has a thickness generally of about 0.125 mm to 0.2 mm. The pitch of the outer lead 3b was about 0.5 mm in an early stage but the pitch has become narrower such as 0.4 mm or 0.3 mm since the high-pin count technique has been developed more in recent years.
For assembling a QFP by using the existent usual lead frame as described above, the semiconductor chip is at first bonded to the die pad 2 by using a solder or epoxy type adhesive. Then, an electrode pad on the semiconductor chip and the inner leads 3a are connected by way of lead wires such as fine gold or aluminum wires. Subsequently, the lead frame is mounted to mold tools and a package main body is prepared by extrusion molding of a heat resistant synthetic resin such as an epoxy resin. FIG. 4 is a plane view showing the state of the surface of the lead frame after resin molding in which a solid black portion shows a resin molded portion.
Vents are disposed to the molding tools for smooth injection molding of the resin.
The resin does not leak through the air vents upon molding owing to the appropriate viscosity of the resin but the resin leaks between the leads to the tie bar 6 to form resin flash 10 as shown in FIG. 5.
After completion of the molding, the tie bars 6 connecting the individual outer leads and the suspending leads 4 are cut by using the tool. FIG. 5 is a plan view illustrating the state for the appearance of the lead frame after cutting the tie bars, in which a solid black portion also shows the resin. As shown in FIG. 5, the resin flash still deposits to the outer leads even after the cutting. Then, an operation for removing the resin flash by blowing pressurized water is applied. Subsequently, the outer lead portion is cut and separated from the outer frame 7 and the inner frame 8 by using the trim and form tools to shape the outer leads into a predetermined configuration.
However, in the series of steps described above, as the pitch of the outer Leads is made smaller to less than 0.5 mm, it results in problems that a manufacturing cost for the tools used for cutting in the step of cutting the tie bar is remarkably increased and that a frequency for the occurrence of defect such as bending to the outer lead is increased. In addition, since pressurized water at a higher pressure has to be used in order to remove the resin remaining in the fine gap, there may be a risk of damaging the package main body.